1. Field of the Invention
The present invention relates to a honeycomb structure and a method of manufacturing the honeycomb structure, particularly to a honeycomb structure which is preferably usable as a filter, for example, for trapping particulates in an exhaust gas of an internal combustion engine, boiler or the like or for filtering liquids such as city water and sewage and in which an increase of a pressure loss at a use time is inhibited, and a method of manufacturing the honeycomb structure.
2. Description of the Related Art
There has been increasingly a need for removing particulates and toxic materials from an exhaust gas of an internal combustion engine, boiler or the like in consideration of an influence on an environment. Especially, regulations on removal of particulate materials exhausted from diesel engines have tended to be reinforced in Europe, America, and Japan, and a honeycomb filter in which a honeycomb structure is used has been used in a trapping filter (hereinafter referred to as DPF) for removing the particulate materials. The honeycomb filter in which the honeycomb structure is used has also been used in filtration of liquids such as city water and sewage (see Japanese Patent Application Laid-Open No. 4-301114, for example).
In general, as shown in FIGS. 21(a) and 21(b), the honeycomb filter for use in this purpose comprises a porous body 32 including an inflow end surface 42 and an outflow end surface 44 of a fluid, and a large number of fluid channels 33a, 33b whose sections have quadrangular shapes and which extend to the outflow end surface 44 from the inflow end surface 42. The adjacent fluid channels 33a, 33b are plugged in a one-end portion on an opposite side in such a manner that the inflow end surface 42 and outflow end surface 44 entirely form a checked pattern. In a honeycomb filter 31 constituted in this manner, fluids such as gas and liquid flow into the fluid channels 33b opened in the inflow end surface 42, that is, the fluid channels 33b plugged in the outflow end surface 44, flow through the porous body 32, and are discharged from the adjacent fluid channels 33a, that is, the fluid channels 33a plugged in the inflow end surface 42 and opened in the outflow end surface 44. In this case, a material to be filtered, contained in the fluid, is filtered by the porous body 32, and the filtered material is deposited on the surface of the porous body 32 constituting inner walls of the fluid channels 33b of the porous body 32.
However, when the honeycomb filter 31 using this honeycomb structure is used as a DPF or the like, many deposits such as soot are deposited on opening portions of the fluid channels 33b of the inflow end surface 42, then an area of the opening portion of the inflow end surface 42 decreases, or the opening portion of the inflow end surface 42 is blocked. This has caused problems that the pressure loss of the honeycomb filter 31 increases and that output drop and deterioration of fuel efficiency of the diesel engine are caused.
To solve the problem, as a filter in which the honeycomb structure is used, for example, a particulate filter 77 (honeycomb filter) shown in FIG. 22 has been proposed including partition walls 74 which define fluid channels 70, 71 (passages). The partition walls 74 are formed of porous materials including pores having a predetermined average pore diameter, and the end portions of the partition walls 74 are gathered and mutually and partially connected in such a manner that end-portion openings of the fluid channels 70, 71 have diameters larger than the pore diameters of the pores of the partition walls 74, but constitute small holes 75, 76 having a channel sectional area smaller than that of the original passage (see Japanese Patent Application Laid-Open No. 2003-49627, for example). A particulate filter 80 (honeycomb filter) shown in FIG. 23 has been proposed. The opposite end surfaces in a flow direction of an exhaust gas form a lattice shape in which the adjacent end surfaces are shifted by a half pitch, and two facing side surfaces continued inwardly from each end surface extend to the vicinity of the end surface on the opposite side in a triangular shape whose inner side is narrowed. Gas inflow side cells 82 and gas outflow side cells 83 are surrounded by walls 84 having a predetermined thickness and are accordingly constituted (see Japanese Patent Application Laid-Open No. 2002-317618, for example). As a device in which the honeycomb structure is used, an exhaust gas purification device 89 shown in FIGS. 24(a) and 24(b) has been proposed including a plurality of passages 86 surrounded by lattice-shaped walls 85 in the flow direction of the exhaust gas and whose rear end side and tip side are alternately plugged to form the gas inflow and outflow sides. Plugging portions 87 form protruding portions 88 protruding in a shape thinned toward an upstream side (see Japanese Patent Application Laid-Open No. 2002-309922, for example).
The honeycomb filters and exhaust gas purification device proposed in these related arts (Japanese Patent Application Laid-Open No. 2003-49627, Japanese Patent Application Laid-Open No. 2002-317618 and Japanese Patent Application Laid-Open No. 2002-309922) are formed so as to increase the area of the portion into which fluids to be treated such as the exhaust gas flow, and it is therefore possible to decrease the pressure loss at the use time.
However, for the above-described honeycomb filter, it is difficult to form the portions gathered or bonded in the end surface with good precision, and there has been a problem that gaps are easily formed in the bonded portions and the exhaust gas to be purified is discharged through the gaps of the bonded portions without being purified. There has also been a problem that the bonded portions have low mechanical strengths and are broken by vibration or pressure of the exhaust gas.
Moreover, in the exhaust gas purification device 89 shown in FIGS. 24(a) and 24(b), there are stepped portions 91 between the surfaces of the protruding portions 88 protruding in the end portions of the passages 86 and the tip portions of the lattice-shaped walls 85, and this causes a problem that the flow of a fluid 90 along the surface of the protruding portion 88 is reduced in this stepped portion and that an inflow resistance increases. Since the protruding portions 88 are formed independently of one another, the adjacent passage 86 is not defined between the end surface on the inflow side and the wall 85, and therefore the fluid 90 flowing into the exhaust gas purification device 89 flows in a direction different from an axial direction of the passage 86, for example, over the adjacent passages 86. This has caused a problem that the pressure loss of the end surface on the inflow side increases.